audio service: fix rapid succession of A2DP devices connections

commit dae476f7330 introduced a potential problem when a different BT A2DP
device is connected just after disconnecting one.

Bug: 37499793
Test: check connection to new BT A2DP device when one is already connected.
Change-Id: I8055844822fcc1640cca67fab2cba629c93f7cfa